Single-parent KidsDo Best

Single mums arebetter at raising their kids than two parents-at least in the bird world.Motherzebra finches have to work harder and raise fewer chicks on their own,but they

also produce moreattractive sons who are more likely to get a mate.

The finding showsthat family conflict is as important an evolutionary driving force as ecologicalfactors such as hunting and food supply.______(46)

In evolutionaryterms,the best strategy for any parent in the animal world is to find someoneelse to care for their offspring,so they can concentrate on breedingagain.Soit's

normal for parents totry to pass the buck to each other.But Ian Hartley from the University ofLancaster and his team wondered how families solve this conflict,and how the

conflict itselfaffects the offspring.

______(47)Theycompared single females with pairs,by monitoring the amount of food each parentcollected,and removing or adding chicks so that each pair of birds was

raising fourchicks,and each single mum had two-supposedly the same amount of work.

But single mums,theyfound,put in about 25 per cent more effort than females rearing with theirmate. ______(48)"The offspring suffer some of the cost of thisconflict,"says

Hartley.

The cost does notshow in any obvious decrease in size or weight,but in how attractive they areto the opposite sex.When the chicks were mature,the researchers tested

the"fitness"ofthe male offspring by offering females their choice of partner.______( 49)

Sexual conflict haslong been thought to affect the quality of care given to offspring,sayszoologist Rebecca Kilner at Cambridge University,who works onconflict of parents

in birds."Butthe experimental evidence is not great.The breakthrough here is showing itempirically."

More surprising,saysKilner,is Hartley',S statement that conflict may be a strong influence on theevolution of behaviour,clutch size and even appearance."People have not

really made thatlink,"says Hartley.A female's reproductive strategy is usually thought tobe affected by hunting and food supply.

______(50)______( 46)

A.To avoid beingexploited,mothers with a partner hold back from working too hard if the fatheris being lazy,and it's the chicks that pay the price.

B.To find out,theymeasured how much effort zebra finch parents put into raising their babies.

C.Those males rearedby single mums were chosen more often than those from two一parent families.

D.With two parentsaround,there's always a conflict of interests,which can have a detrimentaleffect on the quality of the offspring.

E.Kilner saysconflict of parents should now be taken into account as well.